It should be William. Wisconsin's statute on SBS,while the definitions would seem to make possession of the shockwave illegal without a federal tax stamp, it also says it does "not apply to any firearm that may be lawfully possessed under federal law". Since this Federal/State thing is so fuzzy I would confirm this with Mossberg's Customer service: 1 (800) 363-3555.
I would check with the local FFL holder who would do your transfer if its Illegal it could be bad for him so he should know the law
Hi William, Yes it is.